The invention relates to a layer stack for transparent substrates, in particular for panes of glass, with at least one metal oxide composite layer produced notably by reactive cathodic sputtering from a metallic alloy target containing Zn and Sn. The substrates on which the layers are deposited may also be made of transparent organic polymers, and may be rigid or flexible. Rigid polymeric substrates may be chosen in the family of polycarbonates or among certain polyurethans. They may be a methylmetacrylate PMMA. Flexible substrates may be chosen for example in polyethylene terephtalate PET, a film which is afterwards laminated with two thermoplastic sheets (in polyvinyl butyral PVB for example) between two gass panes.
The patent applications EP 0183 052 and EP 0226 993 disclose high-transparency low-E layer stacks in which a metallic functional layer, in particular a thin silver layer, is embedded between two dielectric antireflection layers which are the oxidation product of a zinc/tin alloy. These dielectric oxide layers are sputtered using the method of magnetic field-enhanced reactive cathodic sputtering with an oxygen-containing working gas from a metallic target which consists of a Zn/Sn alloy. Depending on the Zn:Sn ratio, the oxide composite layer produced in this way will contain a greater or lesser amount of zinc stannate Zn2SnO4, which gives the layer particularly favourable properties especially in terms of mechanical and chemical stability. Zn:Sn alloys with a Zn:Sn ratio of from 46:54 to 50:50% by weight are preferably used as the target.
In the technical sputter process with industrial coating stacks, the sputtering of Zn2SnO4 layers from Zn/Sn alloy targets is more difficult than the sputtering of pure ZnO or SnO2 layers. This is because, particularly at the start of the sputtering process, the material on the target and on parts of the sputter chamber lead to insulation effects, the consequences of which are defective products and therefore production rejects. Furthermore, alloy targets of this type must be operated with reduced sputtering rates, that is to say with reduced electrical power, because the target alloy has a lower melting temperature than the melting temperatures of the two components, especially in the region of the eutectic composition. The cooling of targets of this type must therefore be particularly intense. This can in turn be achieved only with targets of particular design, the production of which is comparatively expensive.
The object of the invention is, on the one hand, to further improve the mechanical and chemical properties of dielectric layers containing zinc stannate and, on the other hand, to reduce the difficulties which occur during the process of sputtering Zn/Sn alloys.
According to the invention, this object is achieved in that the metal oxide composite layer contains one or more of the elements Al, Ga, In, B, Y, La, Ge, Si, P, As, Sb, Bi, Ce, Ti, Zr, Nb and Ta.
It has been shown that, by the addition according to the invention of the said elements, which without exception are among the elements in main and subgroups III, IV and V of the Periodic Table, a considerable improvement is obtained in all the important layer properties, with an improvement in the efficiency during the sputter process as well.
The mixed oxides created by the elements added according to the invention, for example by the addition of Al and Sb, have the qualitative composition ZnO.ZnSnO3.Zn2SnO4.ZnAl2O4.ZnSb2O6 depending on the choice of the amounts of the metals Zn and Sn. On crystallization, some of these oxides from spinel structures, which per se crystallize with particularly dense atomic ordering. The improvements obtained in the layer properties can probably be explained by the particularly high packing density obtained for the spinel structures by the incorporation of the said added elements, while the favorable effect during the sputter process is probably attributable to the increase in the electrical conductivity of the mixed oxides which is obtained by the incorporation of the added elements. Owing to the dense crystal structure, the layers not only have particularly high mechanical and chemical stability, but also hinder diffusion processes into this layer or through this layer. This reduces the risk of the onset of modifications in the said layer or in any other layer of the stack which may be attributable to water molecules and oxygen and Na+ and, where applicable (i.e., when the stack contains Ag layer(s)), Ag+ diffusing in, especially during heat-treatment and storage processes.
For a maximally dense spinel structure, it is particularly favourable if the ionic radius of the added element is not too different from the ionic radius of Zn2+ and Sn4+, which have ionic radii of 0.83 angstrom (Zn2+) and 0.74 angstrom (Sn4+), respectively. This condition is satisfied, in particular, for the elements Al and Sb, with which the ionic radius of Al3+=0.57 xc3x85, and of Sb5+=0.62 xc3x85. On the other hand, as already mentioned, the incorporation of the said added elements into the at least partially crystallized layer increases the electrical conductivity of the oxide build-ups on the anode faces and walls of the coating chambers, as well as on the target surface itself. As a result, the operating times of the target during the sputter process are in turn improved considerably, so that not only an improvement in the layer properties, but also an improvement in the sputter process can be observed.
The amount of added elements according to the invention in the metal oxide composite layer is preferably from 0.5 to 6.5% by weight relative to the total amount of metal.
Compositions of the metal oxide composite layer which have been found to be particularly advantageous are those in which, in each case relative to the total amount of metal, the amount of Zn is from 35 to 70% by weight and the amount of Sn is from 29 to 64.5% by weight. For the production of this metal oxide composite layer, alloy targets having from 50 to 70%, notably 66 to 69% by weight Zn, from 29 to 50%, notably 29 to 32% by weight Sn and from 1 to 4% by weight Al or Sb (notably 1.5 to 3%) are preferably employed.
The metal composite layers according to the invention can in particular be used successfully in partially reflecting layer stacks with a metallic functional layer made of silver. In such layer stacks, they can be used both as a bonding or antireflection layer, as a condensation layer for silver layers deposited on top, as a blocker layer below or above the silver layers and as a sublayer in the region of the bottom and/or top layer of the layer stack.
Illustrative embodiments for layer stacks according to the invention will be described below, the properties respectively achieved being compared with the properties of a corresponding layer stack according to the prior art.
In order to assess the layer properties, ten different tests were carried out on all the samples, namely:
A Cracking Hardness
In this case, a weighted needle is drawn over the layer at a defined speed. The weight under which traces of cracking can be seen is used as the measure of the cracking hardness.
B Cracking Hardness After Storage in Water
Test procedure as in A, but after storing the samples in water at 20xc2x0 C. for 30 min.
C Erichsen Wash Test According to ASTM 2486
Visual assessment
D Water Condensation Test (WCT)
The samples are exposed for 140 h to a temperature of 60xc2x0 C. at 100% relative humidity. Visual assessment.
E Zn2+ Leaching
The measurement is taken using the plate method according to Kimmel et al., Z. Glastechnische Berichte 59 (1986) p. 252 et seq. The test gives information about the hydrolytic resistance of layer stacks containing Zn.
F Ag+ Leaching
The measurement is again taken using the plate method according to Kimmel et al. used to determine the Zn2+ leaching. The result of the measurement gives an analytical gauge of the density of the dielectric layers over the Ag layer.
G Hydrochloric Acid Test
In this case, the glass sample is dipped for 8 min in 0.01 n HCl at 38xc2x0 C. and the % emissivity loss is established.
H Hydrochloric Acid Test, Visual Assessment
The glass sample is dipped as for G in hydrochloric acid. The assessment criterion used is what can be seen on the edge which is immersed.
I EMK Test
This test is described in Z. Silikattechnik 32 (1981) p. 216 xe2x80x9cUntersuchungen zur elektrochemischen Prxc3xcfung dxc3xcnner Metallschichtenxe2x80x9d [Studies of the electrochemical testing of thin metal layers]. It gives information about the passivating quality of the cover layer above the silver layer, and about the corrosion resistance of the Ag layer. The lower the potential difference (in mV) between the layer stack and the reference electrode, the better the layer quality.
K Water Film Test
The layer side of the samples is brought into contact for 24 h with a thin film of water. The test gives information about the storage stability of coated panes of glass stacked in a pile if traces of water enter between the panes of glass. The assessment is made visually.